1. Field of the Invention
This invention relates to catalyst compositions and methods of producing auto exhaust catalysts that make no H.sub.2 S odor from the tail pipe, while still meeting catalytic performance requirement for three-way conversions.
2. Description of the Previously Published Art
With the advent of advanced catalysts for controlling auto exhaust emissions, typically containing a higher than usual level of rare earth oxides, especially ceria, the level of H.sub.2 S emission from tail pipes has significantly increased to the extent that the strong H.sub.2 S odor can no longer be tolerated.
In practice, however, it is rather difficult to develop an auto exhaust catalyst that meets the requirements for both the long-term catalytic performance for three-way conversions as well as H.sub.2 S because of the following fact: The performance of the catalyst which has already been optimized for three-way conversions is bound to be sacrificed by any chemical modification introduced to the existing catalyst to meet the requirement for H.sub.2 S.
In our U.S. Pat. No. 4,591,580 we disclosed a stabilized and doubly promoted platinum group catalyst. The alpha-alumina formation was greatly reduced by stabilizing a transitional alumina support with lanthana or La-rich rare earth oxides while still maintaining high catalytic performance. This catalyst comprises four material components which are (1) support or carrier, (2) stabilizer, (3) promoters which are preferably CeO.sub.2 and alkali metal oxides and (4) catalytic metals. The description of this catalyst did not have any discussion of the H.sub.2 S problem in auto emission control nor did the description provide any suggestion on how to solve the problem. There was disclosed an optional embodiment in which Ni could be added from 0.5 to 20 wt. % NiO. However, that nickel was being added to improve the catalyst efficiency for hydrocarbon conversion and especially as a three-way catalyst (TWC) and there was no attempt made to increase the concentration of the NiO in the outer region of the support.
3. Objects of the Invention
It is an object of this invention to chemically modify the auto exhaust emission control catalyst formulation to virtually eliminate the H.sub.2 S odor while still maintaining adequate catalyst performance for controlling hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NO.sub.x).
It is a further object of this invention to disclose a method whereby a slight and yet effective chemical modification can make a TWC highly efficient in lowering H.sub.2 S emission from the tail pipe while retaining virtually all or adequate catalyst efficiency for three-way conversions.
It is a further object of this invention to add an H.sub.2 S gettering ingredient to an auto exhaust emission control catalyst while maintaining the activity for the control of the other emissions.
It is a further object of this invention to add an H.sub.2 S gettering ingredient to an auto exhaust emission control catalyst in a manner so as to achieve a maximum effect on H.sub.2 S control using a minimal level of H.sub.2 S gettering ingredients.
It is a further object of this invention to add an H.sub.2 S gettering ingredient to an auto exhaust emission control catalyst by controlling the getter penetration depths to approximately 600 microns, preferably to approximately 300 microns, so that the H.sub.2 S getter concentration within this outer layer will be much higher than the average getter concentration throughout the support.
These and further objects will become apparent as the description of the invention proceeds.